Hi,
Scattering-parameters are used in high speed signal analysis to find out the reflection(S11) and insertion(S12 OR S21) losses. It depends on the no of ports we are using. lets take an example that we have 2-port network 1- input 2-output.
In s-parameter Sxy x = output y = input. if S21 then it tells input is at 1-port and output is at 2-port.
The losses are calculated in terms of DB.
say input voltage = 1V
if S11 = -3db then 0.7V will reflect back to the source port which is 1.
if s21 = -3db then only 0.7V will be available at the output port which is 2.
Thanks
Shiva